Pieler Lamp. – This is a German lamp (see Fig. 369), burning a non-luminous spirit, such as spirits of wine; there is a shield, a, which also protects the eye of the observer. When this lamp is put in a mixture of firedamp and air, a very small percentage produces a blue cap; ¼ per cent. can be detected with this lamp.

Clowes Hydrogen Lamp. – This lamp, described in Chapter IX. [see below] (see Fig. 334), is specially constructed for the detection of small percentages of gas; 1/5 per cent. can be clearly detected.

Hydrogen Indicator. – The hydrogen flame has been adopted by Professor Clowes, of Nottingham, and its use described by him in a paper communicated to the Royal Society. For the working of this invention he has a small cylinder of compressed hydrogen, and this is connected by a small pipe with the interior of a safety-lamp (see Fig. 334). This figure shows a lamp as patented by Messrs. Clowes and Ashworth; it has, however, since been improved in many details, protected by a later patent, but the principle remains the same. H is the small steel reservoir containing hydrogen at a pressure of say ioo atmospheres; the escape of the gas can be regulated or stopped by a small screw plug, P. The reservoir can be quickly detached and carried in the breast pocket; the attachment or detachment can be effected in three or four seconds. B is the hydrogen pipe, at the top of which is a small burner, the flame from which is regulated by means of the screw tap to a height of 10 millimetres, or 0.4 inch. R is the oil-reservoir, W the ordinary wick, and G the pricker; T is the tube bringing the air to be tested, as in a Gray safety-lamp; A is the glass.

When a test is to be made, the lamp is used as an ordinary safety-lamp, testing for gas with a full flame. If this reveals the presence of gas, there must be a large percentage, the amount of which may be judged by lowering the flame and observing the height of the cap in the ordinary manner. In case, however, the first test does not reveal the existence of fire-damp, the hydrogen cylinder is attached, and the hydrogen turned on, which lights itself at the oil, flame; the oil flame is now put out by means of the pricker. The height of the hydrogen flame is then regulated, and if there is as much as 1/5 per cent. of fire-damp a cap will be distinctly seen ; with ¼ per cent, of gas, the cap is clear but faint, 17 millimetres high; if there is ½ per cent., the cap is slightly longer, that is 18 millimetres, but is much brighter; with 1 per cent. the cap is 22 millimetres, and with 2 per cent. 31 millimetres, with 3 per cent, 52 millimetres. A millimetre is .04 inch. The above measurements are from experiments made with methane by Professor Clowes. The percentage is judged from the brightness as much as from the height of the cap. The oil flame is relighted at the hydrogen flame after the test. The hydrogen flame cannot be blown out or extinguished accidentally. In order to test for gas in any given place, the lamp is first used in the ordinary way as an oil or spirit-lamp; if this gives no indication, the hydrogen flame is next tried, and the oil flame extinguished. With this lamp as small a quantity of fire-damp as 1/5 per cent, can be clearly detected. The utility of such a means of measuring small percentages of .gas is evident, because if in the main return air-way of a mine there is an apparent percentage of say ¼ or ½ and if some of the returns have a less percentage, then it is evident that in some parts of the mine where the gas is given off the percentage will be much greater, and may be, taking into consideration the dust which may also exist, in dangerous quantities, though undetected by the ordinary safetylamp.